The present invention is generally directed to bar code configurations which are useful for representing alphanumeric data. More particularly, the present invention is directed to bar codes in which the bars exhibit uniform single with dimensions and specific periodicity and which are coded to include a subsequence of timing marks to enhance readability. Even more particularly, the present invention is related to high density bar code systems which are employable during the manufacture and processing of semiconductor wafers used in the fabrication of electronic circuit chips, such as those that are produced in very large scale integrated (VLSI) circuit manufacturing processes.
Bar code fonts for representing symbol data, particularly alphanumeric symbols, are very desirable since they provide a mechanism for machine readability which does not depend upon optical character recognition (OCR) systems. In general OCR systems tend to be more error prone than bar code systems. However, bar code systems are nonetheless susceptible to certain error conditions. One example of this is the partial covering of the bar code with opaque films or the reducing of the contrast ratio which makes it more difficult to distinguish between wide and narrow width bars. As an example, if the scanning speed in a bar code reader is not absolutely constant a long gap of spaces between bars is more likely to produce a reading error. Bar codes which have a large number of blank spaces between bars are particularly subject to this kind of error. For example, in certain bar codes a 3% speed variation is sufficient to produce reading errors. Accordingly, it is seen that it is desirable to be able to construct bar code systems in which there is a significant reduction in sensitivity to scanning speed variation.
A number of bar codes employ bars having a plurality of different widths. However, there are certain disadvantages associated with multi-width bar code fonts. In particular, their density, for example as measured in characters per inch, is not as high as one could obtain in a font which only exhibited a single width bar. Furthermore, in bar code systems employing multiple bar widths, it is necessary that the circuitry discern each bar's width or at least the width ratio between bars. Modulated bar widths also introduce writing problems when scribing is carried out with a pulsed laser to form the image. Wide bars, tat is, bars with widths greater than the width of a laser spot, yield a lower quality bar image and require a much longer writing time. For this reason, modulated bar width codes also pose a greater risk of damage to the wafer because of the increased laser radiation concentration. Furthermore, when pulsed lasers are used to write on semiconductor wafers in dot matrix fashion, there is a tendency for a trench to form which throws off the laser used for reading the imparted signal. Thus, modulated bar width systems tend to introduce readability problems when there is a writing quality problem, bar image degradation or low contrast such as might occur in the identification of semiconductor wafer serial numbers.
Single width bar codes are employed but require the simultaneous use of separate timing marks. The single width bar codes therefore require twice the space, plus dual readers. A typical example of such a code is found on certain envelopes as coded by the United States Postal Service. Dual readers are not only more expensive, but the code that they employ takes up more room on a wafer. This room is much more advantageously given over to a human readable version of the code.
While the present invention is generally directed to bar code reading systems having a wide range of applicability to manufacturing, marketing, sorting and identification functions, it is particularly applicable to the identification of semiconductor wafers. In particular, in the manufacture of very large scale integrated circuit devices, that is, chips produced from processed wafers of material such as silicon, it is necessary to employ a code that can be efficiently written and which is robust under the exigencies of processing in unusually harsh environments. Such codes are required to be robust and satisfy the need for representing the full range of alphanumeric characters, A through Z and 0 through 9. This set constitutes a total of 36 characters. However, the letter "O" is often deleted from the required set leaving a total of 35 characters which usually need to be represented and/or distinguished. Additionally, because of size constraints that exist in the manufacture of semiconductor devices it is very desirable that codes employed exhibit a high density. Thus codes which require a separate set of timing marks are undesirable because of the space that the timing marks require.
Furthermore, codes which are suitable for semiconductor chip processing generally should be easy to write on a wafer during processing and should likewise be easily read during wafer or chip processing. Furthermore, the bar code font employed should be such that it is able to withstand processing conditions, namely exposure to sometimes harsh chemicals and high temperature environments. Furthermore, it is desirable to employ codes which can be written by means of a pulsed laser. Such robust and highly reliable codes are desired for wafer fabrication processes to facilitate automatic wafer handling, processing and process parameter identification. Furthermore, it is seen that codes that are developed for such processes would also possess a wide range of applicability in other areas in which bar codes are presently employed.